This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Lithium based battery systems are a clean and effective way to transport energy in our increasingly energy dependent society. With increasing demands for more power, much research is going into new cell architectures and chemistries, consequently increasing demands for stability, cyclability and safety. As an integral component of the battery, electrolytes lie at the crossroads of these ever-increasing needs. Conventional liquid electrolyte-salt combinations often result in safety issues and cell degradation due to inherent flaws such as dendritic growth and thermal runaway. Solid state electrolytes bring increased functionality to the cell in terms of increased stability and safety. However, in general, ionic transport through solid electrolyte materials is orders of magnitude lower than that through liquid electrolytes. If the ionic conductivity mechanisms and the fabrication of solid-state electrolyte materials capable of conducting Li-ions can be optimized, battery safety, lifetime and capacity can be improved significantly.
A newer class of materials discovered shows much promise as a solid state lithium-ion conductor. The garnet oxide of the stoichiometry Li7La3Zr2O12 (LLZO) offer a relatively high ionic conductivity and good chemical stability over a range of voltages. In the chemical mentioned above the individual numbers are usually, and at least in this disclosure, are referred to as the stoichiometry of the particular component. LLZO exists as two high-temperature polymorphs, a more ordered I41/acdZ tetragonal phase and an Ia-3d cubic phase featuring higher ionic conductivity in a more disordered lithium sub-lattice. A decrease in lithium site occupancy as well as changes in interatomic spacing have been attributed to the improvement of the ionic conductivity for cubic LLZO. Although these materials show promise, high temperatures are typically required to achieve the phase stabilization and densification necessary for the desirable material properties.
Site-specific aliovalent dopants have proven useful for tailoring the electrochemical material properties of LLZO through the modification of lattice spacings and overall garnet stoichiometry, allowing for lower temperature stabilization of the cubic phase. Early examinations of LLZO revealed Al impurities from sintering crucibles migrating through the grain boundaries and eventually into the garnet lattice, displacing Li ions in the structure and introducing Li+ vacancies, preferentially stabilizing the cubic phase over the tetragonal phase. Dopants on the 24c La3+ and 16a Zr4+ sites have also been used to reduce the amount of Li+ in order to maintain charge neutrality for higher valence species substituted for the La3+ and Zr4+ ions. Such studies have indicated that there is an optimum Li+ occupancy to vacancy ratio providing the highest ionic conductivity. Additionally, aliovalent dopants modify the garnet lattice parameter and thereby the geometry of the lithium ion conduction channels. Thus, aliovalent dopants can simultaneously modify the lattice spacing and the stoichiometry of the garnet species, greatly affecting the lithium ion mobility through the structure.
The synthesis plays a key role in determining the properties of solid state ion conductors such as LLZO. Conventional ball milling and sintering techniques without dopants can require as much as 36 hours at 1230° C. for stabilization of the cubic phase and subsequent densification necessary to achieve ionic conductivities on the order of 10−3 S/cm. Sol-gel and polymerized complex fabrication techniques such as the Pechini method have been utilized to create a more homogeneous mixture of precursor materials, reducing the energy consumption required for achieving complete mixing. The above methods are either complex or require higher processing temperature making them energy intensive and unattractive.
Thus, there is an unmet need for methods of fabrication of solid-state electrolytes at lower temperatures and yet possessing lithium-ion mobility through the electrolyte suitable for lithium-based batteries.